1. Field of the Invention
The present invention relates to a method for determining a charge state of a battery in a motor vehicle with the aid of a fluxgate magnetometer.
2. Description of Related Art
Modern motor vehicles have a plurality of subsystems that are dependent on a supply of electrical current. Several such subsystems, for example, an electronic immobilizer or an alarm system, may consume current even if a drive engine of the motor vehicle is not in operation, and the required current is provided by an electrical system battery. Frequently, the above-named subsystems become active only at certain intervals, and spend the remaining time in a power-saving mode. In order to ensure a charge state of the battery which is sufficient for starting the drive engine with the aid of an electric starter motor after a longer period of shutdown of the motor vehicle, the subsystems are controlled as a function of the charge state of the battery. Various methods are known for determining the charge state. Usually they require a determination of the flow of the battery current into the battery and from the battery. From the current flow it is possible to infer whether the battery is being charged or discharged, from which the charge state of the battery may be ascertained.
To avoid galvanic coupling in the case of a current determination of the battery current, a fluxgate magnetometer is used to determine a magnetic field that is induced by the battery current in a conductive element connected to the battery. The battery current is then determined from the magnetic field. For magnetic field measurement, a soft magnetic coil core that is exposed to the magnetic field of the conductive element is driven cyclically into magnetic saturation with the aid of a secondary coil. During certain periods of such a measuring cycle, the current flowing through the coil corresponds to the battery current divided by the transmission ratio.
Published German patent application document DE 42 29 948 A1 shows such a current sensor based on a fluxgate magnetometer. The time periods of each measuring cycle in which the coil core has reached magnetic saturation cannot be used for a determination of the magnetic field or the battery current, so that the fluxgate magnetometer is “blind” during these time periods. In particular in the case of electrical consumers having a periodically varying power consumption, these “blind” time periods may result in incorrect determinations of the battery current.